In tube and sheet heat exchangers, for example those used in a nuclear steam generator, problems are encountered in the form of tubes which become defective or damaged. In the past, these tubes were removed from operation by welding plugs into each end during a maintenance shutdown, and thereafter continuing the later operation of the steam generator with reduced capacity with the remaining tubes in operation. This form of maintenance is satisfactory until so many of the tubes are plugged as to make it economically unfeasible to continue operation at the reduced capacity. Thus, instead of doing the above, the tubes are presently plugged by plugs that can be removed at a later time. These plugs are mechanically expanded into the tubes without any welding being done. When too many tubes have been removed from operation, the unit is shutdown, the plugs are removed and the tubes are inspected and tested to see which of the defective tubes can be effectively repaired, for example by sleeving. If enough tubes can be repaired, the useful life of the steam generator can be greatly extended. The plugs are generally loosened by means of a tool which spirals an electrical arc along the inside surface of the plug wall. As the arc melts a portion of its inner surface, the forces of solidification cause the wall of the plug to contract or shrink back, close to the original size that it was before it was expanded into the defective tube. A gripping mechanism is then used for pulling the plug from the tube end. Care must be take during this action so as not to cause damage to the tubes.